Fiber Bragg gratings (FBGs) are well-known elements used in conjunction with laser transmitters as an in-line filter to stabilize the output wavelength produced by the laser. In one application, an FBG is used with a pump laser source to maintain the output wavelength at the desired 980 nm value required for use in an optical amplifier. An FBG basically consists of a longitudinal variation in the refractive index of the core region in an optical fiber, where the variation is normally formed by exposing a section of stripped fiber to a UV source that illuminates the fiber with interfering beams in a manner that modifies the refractive index profile of the core region of the fiber as a function of the beam interference, forming a pattern of alternating sections of different refractive index (i.e., a “grating”). Once the grating is formed, new layers of jacket and coating material need to be re-applied along the stripped area of the fiber to protect the grating.
Throughout these and other processes, the fiber exiting the laser diode package is subjected to an extensive amount of handling, which may lead to further degradation of the long-term reliability/strength of the fiber (especially in situations where the fiber needs to be tightly coiled to comply with integration packaging requirements). The degradation in the long-term reliability is due, at least in part, to defects introduced in the fiber during the course of creating the grating structure.
Conventional methods of accommodating the presence of these defects include the use of special packaging constraints that maintain the portion of the fiber containing the grating in a straight line so that any bend-induced defects are avoided. Alternatively, special packaging can be created that maintains a large bend radius within the section of fiber containing the grating. Given the lengths of fiber associated with these systems, the demands on “special packaging” may be greater than economically feasible for most applications. Moreover, there is an increasing demand to comply with “small form factor” packaging requirements for all optical components, where these requirements do not easily accommodate the use of relatively long lengths of fiber or large radius fiber coils, as common in the prior art, to provide output wavelength stability from laser sources.